The pelletization of thermoplastic materials is of considerable importance for many applications. Pellets, unlike ingots or bars, readily flow in measuring and dispensing apparatus and the size of pellet charges can be readily controlled to small tolerances. Moreover, unlike powders, they do not form dust and are not ingested by persons working with them. Thus, they provide a highly convenient form for the pacakaging, storage and use of many thermoplastic polymers.
The most advantageous approach for reliably producing pelletized materials at high rates is through the use of underwater pelletizers. A typical underwater pelletizer, such as that shown in U.S. Pat. No. 3,271,821, issued to L. F. Street, comprises a die plate supporting body including an inlet for thermoplastic material, an outlet and an extrusion cavity. A die plate defining a plurality of annularly arranged extrusion orifices is positioned in closing relation to the outlet with the orifices in communication with the cavity. A rotatable knife hub having a plurality of knives secured thereto is positioned for cutting plastic extruded from the orifices into plastic pellets, and a water-tight housing, with water inlet and outlet openings, is secured to the body over the die plate. Means, such as channels for hot fluid, are provided for heating the die plate in order to maintain the plastic in flowing condition as it passes through the orifices.
In operation, hot thermoplastic material is continuously extruded through the orifices of the die plate in the form of hot thermoplastic rods or strands into the water filled housing. As the strands are extruded and while immersed in water, they are cut into short lengths or pellets by the rapidly revolving knives. The resulting pellets are quickly cooled by the water and carried in suspension from the housing to a collection station.
Conventional pelletizers encounter a number of problems. One such problem is that the rapidly rotating hub and cutting knives generate a centrifugal water flow and thereby drive water away from the die plate. The result is vortexing and non-uniform cooling of the heated plate with resulting difficulty in maintaining the heated die plate at the desired uniform temperature at all orifices.
Another problem pertains to a shear and bending movement produced on the extruded strand by the cutting knives. In conventional arrangements, the cutting knives produce a pushing and bending force on the strands while they are being severed. This force can push partially cooled plastic material back into an orifice on the far side of a cut.
Yet, a third problem pertains to the heating of the die plate. In typical heating arrangements hot oil or superheated steam is passed through heating channels in the die plate. In such arrangements there is a substantial temperature differential between the input and output ends of the heating channels with the consequence that orifices near the output ends are much more likely to be blocked or "frozen off" by polymer solidified by the water. Furthermore, the use of steam at high temperatures presents the problem of generating and transmitting high temperature gas at very high pressures. High pressure boilers, couplings and piping are required. Accordingly, there is a need for an improved underwater pelletizer.